Depending on the application for which they are implanted in a patient, implantable medical devices (IMDs) may include a variety of electrical and/or mechanical components. Typically, an IMD includes a rigid housing that houses all of its components, which are generally fragile, to protect the components from forces to which they would otherwise be exposed when implanted within the human body. In order to avoid potentially harmful interactions between the components and bodily fluids, e.g., corrosion, IMD housings are typically hermetically sealed. Many IMD housings are fabricated from titanium because of its desirable rigidity and biocompatibility.
The size and shape of an IMD housing is dependent on the sizes and shapes of the components of the IMD. Large components common to most IMDs include a battery and a circuit board that carries digital circuits, e.g., integrated circuit chips and/or a microprocessor, and analog circuit components. Attempts have been made to reduce the size of the IMD housing by reducing the size of these components, changing the shape of these components, and organizing these components within the IMD housing to avoid empty space within the housing.
However, the functional and safety requirements of IMDs have limited attempts to reduce the size and improve the shape of rigid IMD housings. For example, many types of batteries useful for powering an IMD can emit substances that would be harmful to the patient in which the IMD is implanted and to the other components of the IMD. Consequently, existing IMDs typically use hermetic batteries, e.g., batteries within a hermetically sealed housing or case, as a source of power. However, the need to make the housing or case of the battery hermetic limits the thinness and shapes that the battery may have, e.g., due to need for hermetic feedthroughs and the type of welding required for a hermetic housing or case. These limits to the size and shape that a hermetic battery may have in turn limit the ability of IMD designers to reduce the size and shape of IMD housings.
Consequently, the size, shape and rigidity of IMD housings still greatly limit the locations within the human body where an IMD can be practically implanted. Due to these limitations, an IMD is typically implanted within the abdomen, upper pectoral region, or subclavicular region of a patient. Leads or catheters must be used in order to deliver therapy or monitor a physiological parameter at a location of the body other than where the IMD is implanted. Implantation and positioning of leads and catheters can be difficult and time-consuming from the perspective of a surgeon, particularly where the IMD is located a significant distance from the treatment or monitoring site. Moreover, the increased surgical time, increased surgical trauma, and increased amount of implanted material associated with the use of leads and catheters can increase the risk to the patient of complications associated with the implantation of an IMD.
For example, IMDs that are used to treat or monitor the brain, e.g., to deliver deep brain stimulation (DBS) therapy, are implanted some distance away from the brain, e.g., within the subclavicular region of patients. The long leads that connect the implantable medical device to electrodes implanted within the brain require tunneling under the scalp and the skin of the neck, thereby requiring extensive surgery and a prolonged amount of time under general anesthesia during the implant procedure, as well as increased recovery time. In some cases, tunneling the leads under the scalp and skin of the neck requires an additional surgical procedure under general anesthesia. The lengthy tract along the leads is more susceptible to infection, and the leads can erode the overlying scalp, forcing removal so that the scalp can heal. Further, the long leads running under the scalp and through the neck are more susceptible to fracture due to torsional and other forces caused by normal head and neck movements.